The Chemistry of Ionized Ethyl Glycolate,HOCH 2 CO 2 C 2 H 5 •+,and its Enol Isomer,HOCH=C(OH)OC 2 H 5 *+ . Formation of Ionized and Neutral Trihydroxyethylene

The unimolecular reactions of ionized ethyl glycolate, HOCH₂CO₂C₂H₅^•+, and its enol isomer, HOCH=C(OH)OC₂H₅^•+, have been studied by a variety of techniques including ²H-, ¹³C- and ¹⁸O-labelling experiments, kinetic energy release information, analysis of collision-induced dissociation and neutralization–reionization mass spectra and thermochemical measurements. The metastable ionized enol eliminates C₂H₄ essentially exclusively by β-hydrogen transfer to give (HO)₂C=CHOH^•+, which itself expels H₂O with very high selectivity (∼99%). The metastable ionized keto isomer also eliminates C₂H₄, but minor amounts (∼5%) of competing fragmentations resulting in expulsion of H₂O, HOCO^• and HCO^• are also observed. Moreover, in this case, loss of C₂H₄ no longer involves specific β-hydrogen transfer. This behavior is interpreted in terms of rearrangement of the ionized keto form via a 1,5-H shift to the distonic ion HOCH₂C⁺(OH)OCH₂CH₂^•, which undergoes a further 1,5-hydrogen shift to form HOCH=C(OH)OC₂H₅^•+, from which C₂H₄ is lost to give (HO)₂C=CHOH^•+. The chemistry of these C₄H₈O₃^•+ species, in which unidirectional tautomerism of HOCH₂CO₂C₂H₅^•+ to HOCH=C(OH)OC₂H₅^•+ via two 1,5-H shifts is important, contrasts sharply with the behavior of the lower homologues, HOCH₂CO₂CH₃^•+ and HOCH=C(OH)OCH₃^•+, for which the analogous tautomerism via 1,4-H shifts does not occur. The mechanism for loss of C₂H₄ from HOCH=C(OH)OC₂H₅^•+ is essentially the same as that observed for ionized phenyl ethyl ether in that the reaction proceeds via an exothermic proton transfer in an ethyl cation–radical complex. Neutralization–reionization experiments show that neutral trihydroxyethylene in the gas phase is a remarkably stable species which does not tautomerize to glycolic acid, HO–CH₂–COOH. Using G2(MP2) theory, the enthalpy of formation, ΔH_f²⁹⁸, of the neutral molecule (most stable conformer) was found to be −449.4 kJ mol⁻¹ while that of the corresponding ion was calculated to be 299.6 kJ mol⁻¹.